The present invention relates to a field-effect transistor with a semiconductor body which has a main area, in which at least one source zone and one drain zone are introduced and which is provided with a gate electrode isolated from a channel region between the source zone and the drain zone by an insulator layer.
In semiconductor technology, as is known, the miniaturization of components has been a preferred aim for decades. Thus, in integrated circuits, ever higher integration levels are striven for in order to be able to accommodate as many components as possible on a chip. Transistor structures that manage with a minimal space requirement are also especially sought.
In the course of the miniaturization of components, the xe2x80x9cthird dimensionxe2x80x9d has already been included for some time now, by the introduction of trenches in a semiconductor body, the sidewalls and bottoms of the trenches are utilized as a seat for components. An example that may be mentioned here is VMOS technology or UMOS technology. Specifically with regard to UMOS technology, reference may be made here to IEEE Transactions on Electron Devices, Vol. 41 No. 5, May 1994, pp. 14-18, which describes a MOSFET structure with a U-shaped gate electrode which is formed in a trench of a semiconductor body, and a further trench for making contact with source and body regions of the MOSFET structure.
Although the above-mentioned problem situation, namely the creation of ever smaller structures of semiconductor components, has existed for decades, a satisfactory solution has not been achieved to the present day. Rather, success is achieved again and again in configuring ever smaller structures of individual components.
It is accordingly an object of the invention to provide a field-effect transistor having a high packing density and a method for fabricating it which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which has the smallest possible dimensions according to the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention, a field-effect transistor, including:
a semiconductor body having a main area with isolated trenches formed therein, including a first trench, a second trench and a third trench, the semiconductor body having walls defining each of the trenches;
at least one source zone disposed in the walls defining the first trench;
at least one drain zone disposed in the walls defining the third trench;
a channel region disposed in the walls defining the second trench and disposed between the source zone and the drain zone;
an insulator layer disposed in the semiconductor body; and
a gate electrode isolated from the channel region by the insulator layer.
In the case of the field-effect transistor of the type mentioned in the introduction, the object is achieved according to the invention by virtue of the fact that the source zone, the drain zone and the channel region are disposed in walls of a respective trench provided in the semiconductor body.
In other words, in the field-effect transistor according to the invention, three recesses, so-called xe2x80x9ctrenchesxe2x80x9d, are made in a, for example, p-doped surface of a semiconductor body. The trenches essentially lie in a row next to one another. The middle trench is provided with an oxide wall, while the two lateral trenches have an n-doped edge in the present example. In this case, the doping may be effected by diffusion from the trenches. In this case, the doping zones of the n-doped edges of the lateral trenches make contact with the oxide wall of the middle trench. The interior space of all the trenches is filled with a readily conductive material such as, for example, n+-doped polycrystalline silicon or else a metal such as, for example, aluminum, with the result that three xe2x80x9cplugsxe2x80x9d are present. The middle plug in the middle trench then forms the gate electrode of the field-effect transistor, while the two lateral plugs in the lateral trenches serve as source electrode and drain electrode, respectively. The current channel is routed at the sidewall of the middle plug in the semiconductor material behind the oxide wall.
The field-effect transistor according to the invention requires an extremely small space, since the individual trenches for the source, the drain and the gate electrode can be made extremely small, yet it is nonetheless possible to realize structures having large channel widths, which are given by the depth of the middle plug, and short channel lengths, which are predetermined by half the circumference of the middle trench.
What is essential about the present invention, therefore, is that, for the individual active zones of the field-effect transistor, that is to say the source zone, the drain zone and the channel region, separate trenches having extremely small dimensions are provided and are each xe2x80x9cfilledxe2x80x9d with the corresponding electrodes.
Preferred areas of application of the invention are CMOS-ICs and DRAMS, for example. However, it shall be expressly emphasized that an application of the invention in the case of bipolar structures as well is possible and advantageous. If the middle trench in the example above is provided, in its edge, with p-doping instead of an oxide wall, then an npn transistor is present whose three active zones, namely an emitter zone, a base zone and a collector zone, are formed by the dopings in the walls of three trenches, the respective electrodes being composed of the above-mentioned n+-doped polycrystalline silicon in the respective trenches.
The trenches themselves may have an essentially round or circular cross section. It goes without saying, however, that other configurations of the trenches are also possible. Thus, elliptic trenches or indeed rectangular trenches, etc. are readily conceivable. It should be noted that the channel length in the case of the field-effect transistor is given by half the circumference of the middle trench. A short channel length can thus be realized by using an elliptic cross section, for example, instead of a round cross section for the middle trench, the longitudinal axis of the ellipse running between the center points of the two lateral trenches.
In general, the present invention makes it possible to fabricate semiconductor components, and in particular field-effect transistors, which have submicron scale dimensions.
Other advantageous developments of the invention are described below.
The field-effect transistor according to the invention can, for example, readily be equipped with oxide isolation.
Furthermore, it is possible to allow for instance the doping concentration of the drain zone to rise continuously from xe2x80x9cthe outside inxe2x80x9d, that is to say from the semiconductor body toward the plug made of polycrystalline silicon, thereby giving rise to an xe2x80x9cLDDxe2x80x9d-like improvement in the dielectric strength (LDD=xe2x80x9cLightly Doped Drainxe2x80x9d). It is also possible to introduce a region having an elevated doping concentration underneath individual plugs, for example underneath the middle gate plug. The, for example, n+-doped polycrystalline silicon of the individual plugs may also additionally be used as one or more interconnect planes, as a result of which a further increase in the integration level of a corresponding integrated circuit is achieved. The gate- or source-drain resistances can be reduced by introducing titanium nitride or any silicides in the wall of the plugs made of polycrystalline silicon or in a the center of said plugs.
A particularly advantageous development of the invention relates to the application thereof to SOI technology (SOI=silicon on insulator). In this case, the trenches for the source, the drain and the gate only make contact with an Si island embedded in an insulator, the trenches having cross sections in the form of circle sectors, for example. The trenches for source and drain, however, likewise serve as diffusion sources for arsenic and/or phosphorus, for example, while the middle trench has the oxide wall and accommodates the gate electrode.
In addition, it is also possible to provide a terminal for the semiconductor body itself in a fourth trench or recess, from which a p-type dopant, for example, diffuses in order that the threshold voltage of the field-effect transistor can be set precisely.
Since the trenches themselves are embedded approximately by half or more in insulator material, the actual silicon xe2x80x9cislandxe2x80x9d can be embodied such that it-is particular small in the submicron range, as a result of which packing densities not achieved before can be realized.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a field-effect transistor having a high packing density and a method for fabricating it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.